it depends on what you perceive as grooming
if he's licking and rubbing your hand-
then it's definitely grooming and that means he has loads of trust and respect for you. he acknowledges you as a friend and knows that you will bring no harm to him and this bond will only get better providing maintain high standards of care for your guinea pig.
he he's biting or chewing-
he sees you as a threat, he doesn't want your hand there and he wants to get rid of it . if this is the case, leave him for a few days let him relax and research about how to bond with them, carefully get him out of the cage and feed him some veggies, he might not take them straight away but he will after time. This habbit might never stop, but that's ok if you don't mind, they all have different habbits and personalityies, if it doesn't stop it's probably a sign that he's not mad or angry it's just a habbit!
if it's a combination of both-
he sees you as a fellow guinea pig, ii know this sounds silly but it's pretty much the same thing as the first one
(this may also happen if you are male and have a hairy hand, it could purely be because he is just grooming the hair)
hope this helped xxx